Inductive power supply with duty cycle control

ABSTRACT

An inductive power supply that maintains resonance and adjusts duty cycle based on feedback from a secondary circuit. A controller, driver circuit and switching circuit cooperate to generate an AC signal at a selected operating frequency and duty cycle. The AC signal is applied to the tank circuit to create an inductive field for powering the secondary. The secondary communicates feedback about the received power back to the primary controller. The power transfer efficiency may be optimized by maintaining the operating frequency substantially at resonance, and the amount of power transferred may be controlled by adjusting the duty cycle.

This application claims the benefit of U.S. Provisional Application No.61/019,411, filed on Jan. 7, 2008.

The present invention relates to inductive power and more particularlyto a system and method for wirelessly supplying power.

BACKGROUND OF THE INVENTION

In recent years, wireless power supply systems have received increasedattention because of some of their benefits over traditional wired powersupply systems. Some more basic wireless power supply systems arespecifically designed to charge a particular device, which can helpminimize power transfer efficiency issues. Other wireless power supplysystems attempt to account for misalignment, charge different remotedevices and provide different amounts of power. In these systems,maintaining an acceptable power transfer efficiency can be difficult.

Some wireless power systems adjust the operating frequency of an ACsignal across the tank circuit closer to or further from resonance toincrease or decrease the amount of power delivered to the remote device.Other wireless power systems adjust the resonant frequency of the tankcircuit closer to or further from the operating frequency. One issuewith these systems is that the power transfer efficiency between theinductive power supply and the remote device is a function of how closethe operating frequency is to resonance. So, while adjusting theoperating frequency or resonant frequency can provide some control overthe amount of power delivered to the remote device, it may come at thecost of decreased power transfer efficiency.

Other wireless power supplies use a fixed operating frequency andinstead adjust the rail voltage, duty cycle, or phase of the AC signalacross the tank circuit to increase or decrease the amount of powerdelivered to the remote device. One issue with this is that in order forthe power transfer efficiency to be acceptable, the inductive powersupply and remote device may need to be precisely aligned andspecifically designed to work with each other.

SUMMARY OF THE INVENTION

The present invention provides an inductive power supply that maintainsresonance and adjusts duty cycle based on feedback from a secondarycircuit. In one embodiment, the inductive power supply includes aprimary controller, a driver circuit, a switching circuit, and a tankcircuit. The controller, driver circuit and switching circuit cooperateto generate an AC signal at a selected operating frequency and dutycycle. The AC signal is applied to the tank circuit to create aninductive field for powering the secondary. The secondary communicatesfeedback about the received power back to the primary controller. Thepower transfer efficiency may be optimized by maintaining the operatingfrequency substantially at resonance, and the amount of powertransferred may be controlled by adjusting the duty cycle.

In one embodiment, the secondary circuit includes a secondary, arectifier, a switch, a load, a sensor, a secondary controller, and acommunication means. A voltage and/or current sensor detectscharacteristics about the power which are transmitted back to theprimary controller using the communication means. Optionally,over-voltage and over-current protection may be provided. If a faultcondition is detected the load is disconnected using the switch.

In one embodiment, a process for inductively powering a load bymaintaining substantial resonance and adjusting duty cycle is provided.Initially an operating frequency and duty cycle are set to an acceptablevalue. The initial operating frequency is determined by sweeping a rangeof frequencies and selecting the operating frequency which provided thehighest power transfer efficiency. The initial duty cycle is set to arelatively low value, such as 20%, to ensure that too much power is notdelivered to the secondary. Once the initial values have been set, theinductive power supply enters a continuous process of adjusting theoperating frequency to maintain substantial resonance and adjusting theduty cycle depending on whether the amount of power is too high or toolow or temperature is too high.

The present invention provides a simple and effective system and methodfor providing a selected amount of wireless power while maintaining ahigh transfer efficiency. Adjustment of duty cycle provides anotherlevel of control of wireless power transfer, one which can be used tofine tune the amount of power provided to a secondary. Additionally, theability to adjust the amount of power being transferred whilemaintaining substantial resonance results in fewer overall losses andeasier fulfillment of specified power requirements.

These and other objects, advantages, and features of the invention willbe readily understood and appreciated by reference to the detaileddescription of the current embodiment and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an inductive power supply.

FIG. 2 is a block diagram of a secondary circuit.

FIGS. 3A-3D together are a circuit diagram of an inductive power supply.

FIG. 4 is a circuit diagram of a secondary circuit.

FIG. 5 is a flowchart of a process to maintain resonance and adjust dutycycle.

FIG. 6 is a flowchart of a process to adjust the operating frequency tomaintain resonance.

FIG. 7 is an exemplary graph showing frequency versus power transferefficiency.

FIG. 8 is a timing diagram showing a varying duty cycle.

DESCRIPTION OF THE CURRENT EMBODIMENT I. Overview

An inductive power supply or primary circuit in accordance with anembodiment of the present invention is shown in FIG. 1, and generallydesignated 100. The primary circuit 100 includes a primary controller110, a driver circuit 111 including a pair of drivers 112, 114, aswitching circuit 115 including a pair of switches 116, 118, a tankcircuit 120 a primary sensor 122 and an optional wireless receiver 124.The primary controller 110, driver circuit 111 and the switching circuit115 together generate an AC signal at a selected frequency and selectedduty cycle that is applied to the tank circuit 120 to create aninductive field for transferring power wirelessly to a secondarycircuit. A secondary circuit in accordance with an embodiment of thepresent invention is shown in FIG. 2, and generally designated 200. Thesecondary circuit 200 may include a secondary 210, a rectifier 212, aswitch 214, a load 216, a current sensor 218 or voltage sensor 220, asecondary controller 222, a signal resistor 224 for communicating usingreflected impedance and an optional wireless transmitter 226.

In operation, an embodiment of the process for adjusting the duty cycleis shown in FIG. 5, the initial operating frequency is set substantiallyat resonant frequency 504 and the initial duty cycle is set at arelatively low value 506. The primary controller continuously adjuststhe operating frequency 508 to maintain substantially resonant frequencyand continuously determines if the amount of power being transferred istoo high 510. If too much power is being provided or temperatures areabove a preset threshold then the duty cycle is decreased 514. If toolittle power is being provided then the duty cycle is increased 512.Various conditions may temporarily or permanently reduce or halt thepower transfer.

II. Inductive Power Supply

The present invention is suitable for use with a wide variety ofinductive power supplies. As used herein, the term “inductive powersupply” is intended to broadly include any inductive power supplycapable of providing power wirelessly. The present invention is alsosuitable for use with “adaptive inductive power supplies.” As usedherein, the term “adaptive inductive power supply” is intended tobroadly include any inductive power supply capable of providing powerwirelessly at a plurality of different frequencies. For purposes ofdisclosure, the present invention is described in connection with aparticular adaptive inductive power supply, shown in FIGS. 3A-3D andgenerally designated 300. The illustrated adaptive inductive powersupply 300 is merely exemplary, however, and the present invention maybe implemented with essentially any inductive power supply that can bemodified to provide inductive power at varying duty cycles.

In the illustrated embodiment, the adaptive inductive power supply 300generally includes a primary controller 310, a low voltage power supply312, memory 314, a driver circuit 316, a switching circuit 318 a tankcircuit 320, a current sensor 322, a filter 324 and optionally awireless receiver 326. In operation, the primary controller 310, drivercircuit 316 and switching circuit 318 apply power to the tank circuit320 to generate a source of electromagnetic inductive power at aselected frequency and a selected duty cycle.

The primary controller 310 of the illustrated embodiment includes twomicrocontrollers, one to control the frequency and one to control theduty cycle. The frequency microcontroller may be a microcontroller, suchas a PIC24FJ32GA002, or a more general purpose microprocessor. The dutycycle microcontroller may be a microcontroller, such as a dsPIC30F2020,or a more general purpose microprocessor. In alternative embodiments,the primary controller 310 may be implemented using a singlemicrocomputer, FPGA, analog or digital circuit. The driver circuit 316may be discrete components, as shown in FIG. 3D, or they may beincorporated into the primary controller 310. An oscillator (not shown)may be included within the primary controller 310.

The primary circuit 300 may also include a low voltage power supply 312for supplying low voltage power to the primary controller 310, thedriver circuit as well as any other components requiring low voltagepower for operation. In the illustrated embodiment the low voltage powersupply 312 provides scales the input voltage to 3.3 volts. Inalternative embodiments, a different voltage may be provided.

In the current embodiment, the various components of the primary circuit310 collectively drive the tank circuit 320 at a frequency and dutycycle dictated by the primary controller 310. More specifically, theprimary controller 310 controls the timing of the driver circuit 316 andswitching circuit 318. The timing refers to both the frequency and dutycycle of the signal being generated. Frequency as it is being used hererefers to the number of repetitions per unit time of a completewaveform. Duty cycle refers to the proportion of time during which thewaveform is high compared to the total amount of time for a completewaveform. Thus, a square wave as shown in FIG. 8, may be described byits frequency and its duty cycle. Further, the duty cycle may beadjusted while maintaining the same frequency and the frequency may beadjusted while maintaining the same duty cycle. The driver circuit 316of the illustrated embodiment includes two separate drivers and mayinclude additional circuit components to boost and filter the signal.For example, in the current embodiment, the signal is boosted to 20volts, without effecting the timing of the signal.

The switching circuit 318 includes two switches. In the currentembodiment, the switches are implemented as MOS field effecttransistors. In alternative embodiments, other circuit components may beused to implement the switching circuit. Additionally, depending onpower requirements MOSFETs with different characteristics may beimplemented during manufacture. In some embodiments, multiple sets ofswitches may be provided on the circuit board, allowing one set ofswitches to be soldered at the time of manufacture based on theparticular power requirements of that application.

In one embodiment, the switching circuit 115 includes two separateswitches 116, 118 that are switched on at the same frequency, but out ofphase with each other. FIG. 8 illustrates the timing for one embodimentof such a switching circuit. In FIG. 8, both switches have the same dutycycle, but are shifted in time from each other by half of the period ofthe switching waveform. In alternative embodiments, each switch may havea different duty cycle and they the switches may be shifted in time adifferent amount from each other. That is, half period separation andsimilar duty cycle are desirable, but unnecessary, for the switchesbecause it may result in increased power transfer efficiency from theinductive power supply to the remote device.

The tank circuit 320 generally includes the primary and a capacitor. Theprimary of the current embodiment is an air-core coil inductor. A coredinductor can also be used if the proper considerations are made forspatial freedom, monitoring overall power, and feedback. The capacitanceof the capacitor may be selected to balance the impedance of the primarycoil at anticipated operating parameters. In the current embodiment,although three tank capacitors are shown, all three capacitors need notnecessarily be soldered into the circuit at the time of manufacture. Aninductive power supply may be fabricated which at the time of solderingcan have an appropriate capacitance value selected by soldering orswitching different capacitors into the circuit. The tank circuit 320may be either a series resonant tank circuit (as shown in FIG. 3D) or aparallel resonant tank circuit (not shown). The present invention may beincorporated into the adaptive inductive power supply shown in U.S. Pat.No. 6,825,620, which is incorporated herein by reference. As anotherexample, the present invention may be incorporated into the adaptiveinductive power supply shown in U.S. Patent Application PublicationUS2004/130916A1 to Baarman, which is entitled “Adapted Inductive PowerSupply” and was published on Jul. 8, 2004 (U.S. Ser. No. 10/689,499,filed on Oct. 20, 2003), which is also incorporated herein by reference.Further, it may be desirable to use the present invention in connectionwith an adaptive inductive power supply capable of establishing wirelesscommunications with the remote device, such as the adaptive inductivepower supply shown in U.S. Patent Application Publication US2004/130915A1 to Baarman, which is entitled “Adapted Inductive PowerSupply with Communication” and was published on Jul. 8, 2004 (U.S. Ser.No. 10/689,148, filed on Oct. 20, 2003), which is incorporated herein byreference. Further yet, it may be desirable to use the present inventionwith a printed circuit board coil, such as a printed circuit board coilincorporating the invention principles of U.S. Ser. No. 60/975,953,which is entitled “Printed Circuit Board Coil” and filed on Sep. 28,2007 by Baarman et al, and which is incorporated herein by reference inits entirety. In other alternative embodiments, the inductor may beimplemented as a multi-tap inductor and/or the capacitors may beimplemented as a switched capacitor bank that may be used todynamically, before or during use, alter the resonance of the primarycircuit, for example, as described in U.S. Pat. No. 7,212,414, which isentitled “Adaptive Inductive Power Supply” and issued May 1, 2007, toBaarman, which is herein incorporated by reference.

In certain modes of operation, the primary controller 310 may establishthe operating frequency as a function of input from the current sensor322. The controller 310, in turn, operates the driver circuit 318 at thefrequency established by the primary controller 310. The driver circuit316 provides the signals necessary to operate the switching circuit 318.As a result, the switching circuit 318 provides AC (alternating current)power to the tank circuit 320 from a source of DC (direct current)power. In an alternative embodiment, the operating frequency isestablished from a separate communication link, such as the wirelessreceiver 326, implemented in the current embodiment as an IR receiver.

The primary controller 310 may also establish the duty cycle as afunction of input from the current sensor 322. Planned shunting of thesignal resistor on the secondary, which will be described in more detailbelow, may be used to provide information to the primary using reflectedimpedance detected with the current sensor 322. Alternatively, the dutycycle may be established using a separate communication link, such asthe wireless receiver 326, implemented in the current embodiment as anIR receiver. This could also be near field or other RF communicationchannels.

In the illustrated embodiment, the current sensor 322 is a currenttransformer having a primary coil connected to the tank circuit and asecondary coil connected to the primary controller 310. In the currentembodiment, the current sensor 322 includes circuitry to adjust the gainof the output of the current sensor to accommodate the ranges acceptedby the primary controller 310. Further, the amount of gain may beadjusted by the primary controller 310 by applying a signal to theswitch. The inductive power supply 300 may include conditioningcircuitry 324 for conditioning the current transformer output before itis supplied to the primary controller 310. In the current embodiment,the conditioning circuitry 324 is a 5K Hz 2-pole filter. Although theillustrated embodiment includes a current transformer for sensing thereflected impedance of the secondary or remote device, the inductivepower supply 300 may include essentially any alternative type of sensorcapable of providing information regarding reflected impedance from thesecondary 400. Further, although the current sensor 322 of theillustrated embodiment is connected directly to the tank circuit, thecurrent sensor (or other reflected impedance sensor) can be located inessentially any location where it is capable of providing readingsindicative of the reflected impedance.

In the illustrated embodiment, the inductive power supply 300 furtherincludes a memory 314 capable of storing information relating to theoperating parameters of a plurality of secondaries 400. The storedinformation may be used to permit the inductive power supply 300 to moreefficiently power the secondary 400 and more readily recognize faultconditions. In some applications, the inductive power supply 300 may beintended for use with a specific set of secondaries 400. In theseapplications, the memory 314 includes the unique resonant frequency (orpattern of frequencies) for each secondary 400, along with the desiredcollection of associated information, such as maximum and minimumoperating frequencies, current usage and minimum and maximum duty cycle.The memory 314 may, however, include essentially any information thatmay be useful to the inductive power supply 300 in operating thesecondary 400. For example, in applications where it is desirable toestablish wireless communications with the secondary 400, the memory 314may include information regarding the wireless communication protocol ofthe remote device 400.

III. Secondary Circuit

The present invention is intended for use with a wide variety of remotedevices or secondaries of varying designs and constructions. It isanticipated that these various remote devices will require power atvarying frequency and will have different power requirements.

For purposes of disclosure, one embodiment of a secondary circuit 400 isshown in FIG. 4. In the embodiment of FIG. 4, the secondary circuit 400generally includes a secondary 410 for receiving power from theinductive power supply 300, a rectifier 414 (or other components forconverting AC power to DC), a low voltage power supply 412 that scalesthe received power to operate the secondary controller 428, conditioningcircuitry 416, 426 to remove ripple in the signal, current sensor 418,voltage sensor 422, switch 420, load 424, secondary controller 428, asignal resistor 432 and an optional wireless transmitter 430. Inoperation, the rectifier 414 converts the AC power generated in thesecondary 410 to DC power, which is typically needed to power the load.Alternatively, multiple secondary coils receiving power on differentphases can be used to reduce the ripple voltage. This is referenced inApplication Ser. No. 60/976,137, entitled Multiphase Inductive PowerSupply System to Baarman et al, which is herein incorporated byreference. Multiple primary coils may be desired to transmit power ondifferent phases in such an embodiment. In one embodiment, the load is acharging circuit (not shown) for a battery. Charging circuits arewell-known and are widely used with a variety of rechargeable electronicdevices. If desired, the charging circuit may be configured to bothcharge a battery (not shown) and/or power the load 424. In alternativeembodiments the rectifier may be unnecessary and AC power may beconditioned to be used to power the load.

The current sensor 418 detects the amount of current in the receivedpower and provides that information to the secondary controller 428. Thevoltage sensor 422 detects the amount of voltage in the received powerand provides that information to the secondary controller 428. Althoughthe illustrated embodiment includes both a voltage sensor 422 and acurrent sensor 418, only one is necessary. By sensing the voltage and/orcurrent in the secondary circuit and knowing the voltage and/or currentprovided by the primary circuit, the primary controller can calculatethe power transfer efficiency. By sweeping a range of operatingfrequencies, noting the power transfer efficiency at each frequency, theoperating frequency closest to resonance can be determined—itcorresponds with the operating frequency that yields the best powertransfer efficiency. In addition, the voltage and current sensors 418,422 can be used in conjunction with a protection algorithm in thesecondary controller 428 to disconnect the load 424 if a fault conditionis detected. This concept is described in more detail in U.S. patentapplication Ser. No. 11/855,710 entitled System and Method forInductively Charging a Battery to Baarman et al, which was previouslyincorporated by reference.

The secondary controller 428 may be essentially any type ofmicrocontroller. In the illustrated embodiment, the secondary controller428 is an ATTINY24V-10MU microcontroller. The secondary controller 428generally includes an analog to digital converter, and is programmed toprocess the voltage and/or current readings and transmit them to theprimary controller 310 of the inductive power supply 300. Themicroprocessor may also include other code unrelated to the frequency orduty cycle control processes.

Communication of the sensed voltage and/or current in the secondary maybe transmitted to the primary controller 310 in a variety of ways. Inthe illustrated embodiment, the information may be transmitted using thesignal resistor 432 or the wireless transmitter 430.

In one embodiment, signal resistor 432 may be used to send informationto the primary controller 310. The use of a signal resistor 432 toprovide communication from the secondary to the primary was discussed inU.S. patent application Ser. No. 11/855,710 entitled System and Methodfor Inductively Charging a Battery to Baarman et al, which is hereinincorporated by reference. The signal resistor 432, when shunted, sendsa communication signal that signifies an over-current or over-voltagestate. When the resistor is shunted, the peak detector on the primarycircuit is able to sense the over-voltage/over-current condition and actaccordingly. The signal resistor 432 of the present invention may beshunted systematically to communicate additional data to the primarycontroller 310. For example, a stream of data could represent the sensedcurrent and/or sensed voltage. Alternatively, the signal resistor couldbe used solely in the previously described way as anover-voltage/over-current transmitter or it could be removed entirely.

Use of a wireless transmitter or transceiver was previously described inU.S. Patent Application Publication US 2004/130915A1 to Baarman, whichis entitled “Adapted Inductive Power Supply with Communication” that waspreviously incorporated by reference. Specifically, the use of WIFI,infrared, blue tooth, cellular or RFID were previously discussed as waysto wirelessly transmit data from a remote device to an inductive powersupply. Further, communication using the induction coils and a powerline communication protocol was discussed. Any of these methods oftransmitting data could be implemented in the present invention in orderto transfer the desired data from the secondary to the primary.

IV. Operation

General operation of the primary circuit 100 and secondary circuit 200is described in connection with FIG. 5.

In this embodiment, the primary circuit determines and sets the initialoperating frequency 504. Typically, the goal of setting the initialoperating frequency is to set it as close to the resonant frequency aspossible, which varies depending on many different factors including,among other things, the orientation and distance between the primarycircuit and secondary circuit. In the current embodiment, a simplefrequency sweep is used to determine where to set the initial operationfrequency. Specifically, in this embodiment, the range of validfrequencies is swept and the power transfer efficiency at each frequencyis noted. The step between frequencies may vary, but in the currentembodiment, the frequency is swept between 70 k Hz and 250 k Hz at stepsof 100 Hz. Once the entire range of frequencies has been swept, theoperating frequency that yielded the highest power transfer efficiencyis selected as the initial operating frequency. The operating frequencythat yielded the highest power transfer efficiency indicates that it isthe closest frequency to resonance. Further steps at a finer frequencyresolution can facilitate even further tuning. Other methods fordetermining the initial operating frequency may be used in alternativeembodiments. For example, an initial operating frequency may be selectedbased on known primary and secondary component. Further, modificationsto the sweeping process may include dynamic step adjustment proportionalto the power transfer efficiency. In yet another alternative embodiment,the sweep may be performed dynamically so that only the power transferefficiency value for the current frequency and the frequency with thehighest power transfer efficiency are stored. As the sweep progresses,each value is checked against the highest stored value and replaces itonly if it is higher.

In the embodiment described in FIG. 5, the primary circuit sets theinitial duty cycle 506. The duty cycle corresponds with the amount ofpower transferred with each cycle. The higher the duty cycle, the morepower transferred per cycle. In the current embodiment, the initial dutycycle is set at 20%, which is considered low enough to not riskover-powering the remote device, but is high enough such that enoughpower is transferred to power the secondary circuitry. In alternativeembodiments a different initial duty cycle may be set based on theapplication or any number of other factors.

The adjust operating frequency step 508 is a multi-step process whichensures that the operating frequency is being maintained substantiallyat resonance. FIG. 6 describes one embodiment of this process in moredetail. In the described embodiment, the operating frequency isincreased by a pre-selected amount, referred to as a step up. Theadjustment is allowed to propagate through the system and the powerefficiency is checked 604. If the power efficiency increased then thesystem was not substantially at resonance and the operating frequency isstepped up again. This process continues until the power efficiencyeither decreases or stays the same. Once that occurs, the operatingfrequency is stepped down 608. The power efficiency is checked 608. Ifthe power efficiency increases then the operating frequency is steppeddown again, until the power efficiency stays the same or decreases. Thefinal step is to step up the operating frequency 610 to get back to theoperating frequency with the peak power efficiency. This is merely oneembodiment of a process to maintain the operating frequencysubstantially at resonance. Any other process could be used to maintainthe operating frequency substantially at resonance.

One reason that the operating frequency is stepped up and stepped downcan be explained by looking at an exemplary graph of operating frequencyvs. power efficiency, shown in FIG. 7. As can be seen, there are severalpeaks of power efficiency over the range of operating frequencies shown.The initial sweep of frequencies sets the operating frequency to theresonant frequency, i.e. the highest peak on FIG. 7. Each time theadjustment comes, although the operating frequency has not changed, thepower efficiency values may have changed as a result in any number offactors, most notably movement of the secondary. Typically, the changein the graph is merely a slight shift, meaning that the optimumoperating frequency may be a few steps in either direction. This is whythe current embodiment steps up and steps down. If the first step upleads to a decrease in power efficiency transfer, the processimmediately steps down until. If stepping down also leads to a decreasein power efficiency transfer then it is evident that no adjustment isnecessary and the operating frequency was already at resonant frequency.In an alternative embodiment an analog circuit could be used to directlydetermine how far off resonance the system is, causing the controller toreact directly to the proper frequency. A phase comparator is one suchcircuit.

In the current embodiment, the operating frequency is adjusted with eachiteration, however, in alternative embodiments, the operating frequencymay be adjusted less frequently or only when an event triggers that itshould be adjusted. For example, if a motion detector on the secondaryindicates movement or a change in orientation of the secondary. Or, forexample, if there is a sharp decrease or increase in the amount of powerprovided to the secondary.

The next step is to determine if the amount of power being received bythe secondary is too high 510. If the amount of power being received istoo high then the duty cycle of the power being transferred is reduced514. If the amount of power being received is not too high then the dutycycle of the power being transferred is increased 512. In the currentembodiment, the duty cycle should not exceed approximately 49% in orderto reduce the risk of causing a short circuit. In the currentembodiment, after the duty cycle is adjusted, up or down, the operatingfrequency is re-adjusted 508. As explained above, duty cycle refers tothe “switch on time” or the proportion of time during which the waveformis high compared to the total amount of time for a complete waveform. Anexemplary graph illustrating a signal with a varying duty cycle is shownin FIG. 8. The graph depicts a graph of time vs. current. The solid linerepresents the waveform generated by the primary circuit with thecurrent duty cycle. The dashed line represents what a waveform wouldlook like with an increased duty cycle. The dash-dotted line representswhat a waveform would look like with a decreased duty cycle. Note thatbecause the duty cycle is being increased symmetrically and decreasedsymmetrically, the frequency of the waveform does not change with theadjustment in duty cycle. It is worth noting that in some embodiments,during operation, the frequency may not be adjusted, while duty cycleadjustments continue to take place.

Duty cycle may be stepped up or down by a pre-selected amount. In thecurrent embodiment, the step up and step down amounts are static andequal. However, in alternative embodiments, the step amounts may bedynamic and different. For example, in battery charging applications itmay be beneficial to decrease duty cycle in large steps and increaseduty cycle in small steps. Various batteries require different chargingalgorithms and the duty cycle control may be used to provide the correctbattery charging profile. In another example, the duty cycle may bestepped up or down proportional to the amount of power demanded by thesecondary. The amount of power demanded by the secondary can bedetermined by reading the current and/or voltage sensor. Where there isa small change in the readings, a small change in duty cycle may beimplemented and where there is a large change in the readings, a largechange in duty cycle may be implemented.

In one embodiment, there are built-in delays between the changes inoperating frequency and changes in duty cycle. These delays can accountfor any phase issues that may arise because of the speed at which theoperating frequency or duty cycle is being changed.

This process continues as desired or until the power supply is turnedoff, the secondary is removed, or in the case of charging a battery,when the battery is fully charged.

The primary circuit may adjust the duty cycle depending on the demandsof the secondary. For example, in one embodiment, one goal may be tomaintain a certain amount of voltage or current in the secondary. Usingfeedback from the secondary, such as the sensed voltage and/or current,the operating frequency may be adjusted to ensure optimum power transferefficiency by ensuring operation at substantially resonant frequency andthe duty cycle may be adjusted to provide additional or less power tomeet the desired goal.

The above description is that of the current embodiment of theinvention. Various alterations and changes can be made without departingfrom the spirit and broader aspects of the invention.

The invention claimed is:
 1. An inductive power supply for providingpower wirelessly to a remote device, said inductive power supplycomprising: a primary circuit for generating a signal at an operatingfrequency, said primary circuit being capable of adjusting a duty cycleof said signal to allow variation in an amount of power transferred tothe remote device at said operating frequency; a tank circuit inelectrical communication with said primary circuit, wherein said primarycircuit applies said signal to said tank circuit to transfer an amountof power to said remote device; said inductive power supply configuredto receive feedback from said remote device; said primary circuit beingconfigured to control said operating frequency of said signal inresponse to said feedback to optimize power transfer efficiency betweensaid inductive power supply and said remote device; and said primarycircuit being configured to control said duty cycle of said signal inresponse to said feedback to control said amount of power transferred tosaid remote device at said operating frequency.
 2. The inductive powersupply of claim 1 wherein said primary circuit maintains said operatingfrequency of said signal substantially at resonance.
 3. The inductivepower supply of claim 1 wherein said primary circuit continuouslyadjusts said operating frequency to maintain substantial resonance andcontinuously adjusts said duty cycle based on a comparison between saidamount of power transferred to said remote device and a threshold. 4.The inductive power supply of claim 1 wherein said primary circuitcontrols said duty cycle of said signal according to at least one of abattery charging profile and a demand by said remote device.
 5. Theinductive power supply of claim 1 wherein said primary circuit includes:a primary controller; a driver circuit in electrical communication withsaid primary controller; a switching circuit in electrical communicationwith said driver circuit; and a sensor for sensing reflected impedanceof said remote device, wherein said sensor is in electricalcommunication with said tank circuit and said primary controller.
 6. Theinductive power supply of claim 5 wherein said switching circuitincludes a pair of switches, wherein each switch is switched on at saidduty cycle and at said operating frequency, but out of phase with eachother; and wherein, in response to said feedback, said primarycontroller controls said operating frequency of each of said switches;and wherein, in response to said feedback, said primary controllercontrols said duty cycle of each of said switches.
 7. The inductivepower supply of claim 5 wherein said primary controller adjusts saidoperating frequency of said signal as a function of input from saidsensor.
 8. The inductive power supply of claim 1 wherein said primarycircuit includes a wireless receiver for receiving said feedback fromsaid remote device.
 9. An inductive power supply system comprising: aninductive power supply including: a primary circuit for generating asignal at an operating frequency and for adjusting a duty cycle of saidsignal to control an amount of power transferred at said operatingfrequency; and a tank circuit in electrical communication with saidprimary circuit, wherein said primary circuit applies said signal tosaid tank circuit to transfer an amount of power to said remote device;said inductive power supply configured to receive feedback from saidremote device; a remote device separable from said inductive powersupply for receiving power from said inductive power supply, said remotedevice including: a secondary energized by said inductive field, a loadin electrical communication with said secondary, a sensor in electricalcommunication with said secondary, a secondary controller in electricalcommunication with said sensor, and a communication device in electricalcommunication with said secondary controller for sending feedback tosaid inductive power supply; wherein, in response to said feedback, saidprimary circuit controls said operating frequency of said signal tooptimize power transfer efficiency between said inductive power supplyand said remote device; and wherein, in response to said feedback, saidprimary circuit controls said duty cycle of said signal to control saidamount of power transferred to said remote device at said operatingfrequency.
 10. The inductive power supply system of claim 9 wherein saidprimary circuit maintains said operating frequency of said signalsubstantially at resonance.
 11. The inductive power supply system ofclaim 9 wherein said primary circuit continuously adjusts said operatingfrequency to maintain substantial resonance and continuously adjustssaid duty cycle based on a comparison between said amount of powertransferred to said remote device and a threshold.
 12. The inductivepower supply system of claim 9 wherein said primary circuit controlssaid duty cycle of said signal according to at least one of a batterycharging profile and a demand communicated to said inductive powersupply by said remote device.
 13. The inductive power supply system ofclaim 9 wherein said primary circuit includes: a primary controller; adriver circuit in electrical communication with said primary controller;a switching circuit in electrical communication with said drivercircuit; and a sensor for sensing reflected impedance of said remotedevice, wherein said sensor is in electrical communication with saidtank circuit and said primary controller.
 14. The inductive power supplysystem of claim 13 wherein said switching circuit includes a pair ofswitches, wherein each switch is switched at said duty cycle and at saidoperating frequency, but out of phase with each other; and wherein, inresponse to said feedback, said primary controller controls saidoperating frequency of each of said switches; and wherein, in responseto said feedback, said primary controller controls said duty cycle ofeach of said switches.
 15. The inductive power supply system of claim 13wherein said primary controller adjusts said operating frequency of saidsignal as a function of input from said sensor.
 16. The inductive powersupply system of claim 9 wherein said primary circuit includes awireless receiver and said remote device includes a wirelesstransmitter, wherein said wireless receiver receives said feedback fromsaid wireless transmitter.
 17. A method for transferring power from aninductive power supply to a remote device, said method comprising:setting an initial operating frequency of a signal in the inductivepower supply; setting an initial duty cycle of the signal to set aninitial power level in the inductive power supply; applying the signalto a tank circuit for transferring an amount of power from the inductivepower supply to a remote device; receiving, in the inductive powersupply, feedback from the remote device; adjusting, in response to thefeedback, the operating frequency of the signal to optimize powertransfer efficiency between the inductive power supply and the remotedevice; adjusting, in response to the feedback, the duty cycle of thesignal to control the amount of power transferred to the remote device.18. The method for transferring power of claim 17 wherein said step ofadjusting the duty cycle includes: decreasing the duty cycle of thesignal in response to a determination that the power transferred to theremote device is above a threshold; and increasing the duty cycle of thesignal in response to a determination that the power transferred to theremote device is below a threshold.
 19. The method for transferringpower of claim 17 wherein at least one of said setting an initialoperating frequency and adjusting said operating frequency includessweeping a frequency range, determining an amount of power transferredto the remote device for each operating frequency, and selecting anoperating frequency where the amount of power transferred to the remotedevice is relatively high compared to other frequencies within thefrequency range.
 20. The method for transferring power of claim 17wherein at least one of said setting an initial operating frequency andadjusting said operating frequency includes sweeping a frequency rangeand selecting the operating frequency closest to resonance.
 21. Themethod for transferring power of claim 17 wherein said adjusting of theoperating frequency includes continuously adjusting the operatingfrequency to maintain substantial resonance and said adjusting of theduty cycle includes continuously adjusting the duty cycle based on acomparison between the amount of power transferred to the remote deviceand a threshold.
 22. The method for transferring power of claim 17wherein said adjusting the duty cycle includes adjusting the duty cycleaccording to at least one of a battery charging profile and a demand bythe remote device.
 23. An inductive power supply for a remote devicecomprising: a tank circuit; a primary circuit in electricalcommunication with said tank circuit, said primary circuit configured toapply a signal to said tank circuit; and a receiver configured toreceive communications from the remote device, said receiver inelectrical communication with said primary circuit, said primary circuitconfigured to selectively adjust an operating frequency of said signalas a function of said communications to optimize power transferefficiency from said tank circuit to the remote device, said primarycircuit configured to selectively adjust a duty cycle of said signal asa function of said communications to control an amount of powertransferred to the remote device at said operating frequency, wherebysaid inductive power supply maintains power transfer efficiency throughoperating frequency adjustments and while maintaining appropriate powerlevel though duty cycle adjustments.
 24. The inductive power supply ofclaim 23 wherein said receiver is further defined as a current sensorelectrically coupled to said tank circuit to provide a signal indicativeof current in said tank circuit.
 25. The inductive power supply of claim23 wherein said receiver is further defined as a communication receiver.26. A method for wirelessly supplying power to a remote device,comprising the steps of: placing a remote device in sufficient proximityto an inductive power supply to establish an inductive coupling betweenthe remote device and the inductive power supply; in the inductive powersupply, applying a signal to a tank circuit at a plurality of differentoperating frequencies to wirelessly transfer power to the remote device;in the remote device, taking separate measurements of a characteristicof the power wirelessly received from the inductive power supply foreach of said different operating frequencies; determining an initialoperating frequency based on the separate measurements; and in theinductive power supply, applying a signal to the tank circuit at theinitial operating frequency receiving, in the inductive power supply,communication from the remote device; adjusting, in response to thecommunication, the operating frequency of the signal to optimize powertransfer efficiency between the inductive power supply and the remotedevice; and adjusting, in response to the communication, the duty cycleof the signal to control the amount of power transferred to the remotedevice.
 27. The method of claim 26 wherein said determining stepincludes storing information indicative of a power transfer efficiencyat each of the plurality of different operating frequencies; andselecting the initial operating frequency as the operating frequencyhaving the greatest power transfer efficiency.
 28. The method of claim26 wherein said determining step includes storing information indicativeof the operating frequency having the greatest power transferefficiency; and selecting the initial operating frequency as theoperating frequency having the greatest power transfer efficiency. 29.The method of claim 26 wherein the step of applying a signal to the tankcircuit includes applying the signal at a first power level selected tobe low enough not to risk over-powering the remote device.
 30. Themethod of claim 26 wherein the step of applying a signal to the tankcircuit includes applying the signal at a duty cycle selected to be lowenough not to risk over-powering the remote device.
 31. The inductivepower supply of claim 1 further including a first controller to controlsaid operating frequency and a second controller to control said dutycycle.
 32. The inductive power supply of claim 1 further including amemory storing information relating to operating parameters of aplurality of remote devices.
 33. The inductive power supply of claim 10wherein said information includes for each of the remote devices atleast one of a resonant frequency, a maximum operating frequency, aminimum operating frequency, a current usage, a minimum duty cycle, amaximum duty cycle and wireless communication protocol of the remotedevice.
 34. The method of claim 26 further comprising the step ofdetermining an initial duty cycle based on the separate measurements.35. The method of claim 26 further comprising the step of selecting aninitial duty cycle of at or less than 20% so that too much power is notdelivered to the remote device.